Recent development in technology of optical industry including optical devices, optical communication and displays demands excellence in optical properties of materials. Examples of materials include optical materials and electronic materials such as optical lenses, optical disk substrates, substrates for liquid crystal display elements, substrates for color filters, substrates for organic EL (Electro Luminescence) display elements, substrates for solar cells, touch panels, optical elements, optical waveguides and sealing materials for LED (Light Emitting Diode).
In general, materials for forming substrates for liquid crystal display elements, substrates for color filters, substrates for organic EL display elements, substrates for solar cells and touch panels are often inorganic glass. However, many attempts have been made in recent years to replace glass plates with plastic materials, since glass plates are, for example, fragile, cannot be bended and are not suitable for weight reduction because of their large specific gravity. The optical materials such as substrates for liquid crystal display elements demand high transparency, too, since light passes through these materials.
As materials for forming optical lenses, optical elements, optical waveguides and LED sealing materials, recently, there has been demand for plastic materials with excellent heat resistance that deal with lead-free solders. For example, it is desired that the plastic materials have smaller volume change and lower coefficient of linear expansion upon being heated. In the application of the plastic materials instead of glass plates to optical parts and electronic parts such as optical lenses and optical waveguides, it is important that the shrinkage percent at the time of curing is low for their precise processing.
Conventional materials for forming optical materials are as follows. For example, Patent Literature 1 discloses a component prepared by curing a resin composition with an active energy ray, wherein the resin composition includes an amorphous thermoplastic resin and a bis(meth)acrylate curable with an active energy ray. Patent Literature 1 describes that instead of glass substrates, said component is preferably utilized for optical lenses, optical disk substrates and plastic substrates for liquid crystal displays. However, in Patent Literature 1, the shrinkage percent of said resin composition and the coefficient of linear expansion of said component are not studied; and the transparency of said component can be reduced because of the difference in refractive index between the amorphous thermoplastic resin and a resin obtained by curing the bis(meth)acrylate with an active energy ray.
Patent Literature 2 discloses a curable composition obtained by homogenously dispersing a silica-based condensation polymer in methyl methacrylate or in a bisphenol A-type ethylene oxide-modified (meth)acrylate, wherein the silica-based condensation polymer is prepared by the hydrolysis and condensation polymerization of a specific silane compound in a colloidal silica dispersion system. Furthermore, Patent Literature 2 describes that said composition can provide a cured product excellent in transparency and rigidity and that the cured product is useful in applications such as optical materials. However, in Patent Literature 2, the shrinkage percent of the curable composition and the coefficient of linear expansion of the cured product are not studied.
Generally known methods for reducing the shrinkage percent and the coefficient of linear expansion include a method adding an inorganic filler to a resin composition and a method laminating an inorganic film on a substrate. However, a resin composition containing the inorganic filler, after being cured, gives a cured product (substrate) that has significantly impaired transparency and has no surface smoothness and moreover is easily crackable as a result of the ununiformity caused in the substrate due to poor dispersibility of the inorganic filler.
Laminating an inorganic film on a substrate causes, for example, problems listed below. The problem (2) is caused, for example, by the large difference in terms of shrinkage at the time of curing between the inorganic film and a resin composition which after being cured serves as substrate.
(1) Poor adhesion between the inorganic film and the substrate.
(2) Peeling of the inorganic film from the substrate, or cracking of the substrate.
Patent Literature 3 describes a curable composition prepared by homogenously dispersing a silica-based condensation polymer in a (meth)acrylate, wherein the silica-based condensation polymer is obtained through the hydrolysis and condensation polymerization of a silane compound having a hydrocarbon residue having 1 to 10 carbon atoms in a colloidal silica dispersion, the curable composition being capable of giving a cured product with excellent transparency and rigidity. However, the viscosity and shrinkage percent of the cured product obtained from this curable composition are not studied.
Patent Literature 4 describes a curable composition obtained by homogenously dispersing a silica-based condensation polymer in a bisphenol A-type ethylene oxide-modified (meth)acrylate, wherein the silica-based condensation polymer is obtained through the hydrolysis and condensation polymerization of a specific silane compound in a colloidal silica dispersion system, the curable composition being capable of giving a cured product with excellent transparency and rigidity. In Patent Literature 4, the number of the carbons of an organic group having an ethylenic unsaturated group of the silane compound is limited to 10 or less, i.e., short chains. This makes the hydrophobicity of the colloidal silica insufficient, and blending of the colloidal silica in an amount exceeding 15 percent by weight would cause gelation. Thus, a sufficient amount of silica cannot be added, and the coefficient of linear expansion cannot be expected to be reduced.
Patent Literature 5 discloses a cured product obtained by crosslinking a composite composition obtained by removing an organic solvent in a composition which contains a bifunctional (meth)acrylate having a specific alicyclic structure and colloidal silica dispersed in an organic solvent. However, in the invention described in Patent Literature 5, the dispersibility of the silica in the composite composition and the inhibition of curing shrinkage are insufficient. This literature describes compensating for the dispersibility of the silica and adding a silane compound having an alicyclic structure to the composition in order to reduce the viscosity of the composite composition, but the hydrolysis of this silane compound is significantly slow. Thus, along with the failure to provide economical advantage in terms of production time, it is difficult for the effects compound to exhibit. In Patent Literature 5, there is a mention of coefficient of linear expansion, but the curing shrinkage is large.